MIT researchers have 3D printed and designed a sensory device that responds to mechanical stresses by changing the color of a spot on its surface.

Developed by MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL), the device is inspired by the golden tortoise beetle, an insect whose exterior usually appears golden but turns reddish orange if the insect is poked or prodded – implying that it is mechanically stressed.

“In nature, networks of sensors and interconnects are called sensorimotor pathways,” says team lead Subramanian Sundaram, an MIT graduate student in electrical engineering and computer science. “We were trying to see whether we could replicate sensorimotor pathways inside a 3-D-printed object. So we considered the simplest organism we could find.”

Printable electronics, in which flexible circuitry is deposited on some type of plastic substrate, has been a major area of research for decades. But Sundaram says that the ability to print the substrate itself greatly increases the range of devices the technique can yield.

According to the researchers, a printed substrate could consist of many materials, interlocked in intricate but regular patterns, which in turn broadens the range of functional materials that printable electronics can use.

Printed substrates also open the possibility of devices that, although printed as flat sheets, can fold themselves up into more complex, three-dimensional shapes. Printable robots that spontaneously self-assemble when heated, for instance, are a topic of ongoing research at the CSAIL Distributed Robotics Laboratory, led by Professor Daniela Rus.

The MIT researchers’ new device is approximately T-shaped, but with a wide, squat base and an elongated crossbar. The crossbar is made from an elastic plastic, with a strip of silver running its length; in the researchers’ experiments, electrodes were connected to the crossbar’s ends.

The base of the T is made from a more rigid plastic. It includes two printed transistors and what the researchers call a “pixel,” a circle of semiconducting polymer whose color changes when the crossbars stretch, modifying the electrical resistance of the silver strip.

The transistors and the pixel are made from the same material; the transistors also change color slightly when the crossbars stretch. The effect is more dramatic in the pixel, however, because the transistors amplify the electrical signal from the crossbar.

To build the device, the researchers used the MultiFab, a custom 3D printer developed by associate professor Wojciech Matusik’s group. The MultiFab already included two different “print heads,” one for emitting hot materials and one for cool, and an array of ultraviolet light-emitting diodes. Using ultraviolet radiation to “cure” fluids deposited by the print heads produces the device’s substrate.